Video content services represented by a video streaming service are rapidly increasing, and the data of those services, which require a larger transmission capacity than the data of still image services, have made up a large fraction of the Internet traffic. This makes the transmission capacity within a network increase rapidly, and a technology to maximize the usage efficiency of existing facilities has been strongly required in parallel with adding more transmission paths or transmission devices.
The elastic optical network has been researched that is one of the technologies to maximize the usage efficiency of existing facilities and remarkably improves the frequency usage efficiency within an optical fiber. The elastic optical network is an optical network in which communication is performed selecting an optimum modulation method depending on a transmission distance or a demanded throughput. Selecting an optimum modulation method makes it possible to transmit large-volume data with a minimum frequency band.
In the elastic optical network by which large-volume data can be transmitted with high efficiency as described above, an optical transmitter is used that can select an optimum modulation method depending on a transmission distance or a demanded throughput. Because the optical transmitter used in the elastic optical network adaptively changes the modulation method depending on a transmission distance, it is necessary to make an optical transmitter with maximum performance in variable range; accordingly, it is often the case that the optical transmitter becomes overengineered compared with actual use conditions. Since a conventional optical transmitter uses a single carrier frequency, the performance of the elastic optical network that the bandwidth is variable cannot be utilized sufficiently if there is no solid blocks of band on a frequency axis. For example, Patent Literature 1 discloses an optical transmitter that can utilize the performance of the elastic optical network sufficiently, and Patent Literature 2 discloses an optical node device including a plurality of optical transmitters.
The optical transmitter disclosed in Patent Literature 1 includes a frequency interval setting unit that is disposed between a multi-frequency phase-synchronized light source and an optical modulator and individually adjusts a frequency interval of the multi-frequency phase-synchronized light source for each frequency. Adjusting the frequency interval individually for each frequency makes it possible to apply an optimum modulation rate easily; accordingly, it is possible to utilize sufficiently the performance of an optical transmitter/receiver used in the elastic optical network.
The multi-flow optical node disclosed in Patent Literature 2 includes a plurality of carrier generators or sub-carrier generators and improves the usage efficiency of the optical transmitter by allocating the carriers or the sub-carriers to modulators efficiently through optical multiplexing/demultiplexing means. This enables the identical optical transmitter to accommodate demanded services from a plurality of clients and to transmit the services to a plurality of different routes, so as not to waste a plurality of modulators in the optical transmitter.